The U.S. military closed down its largest detention center on Thursday in Iraq, as it moves to release thousands of detainees or transfer them to Iraqi custody before the end of the year.

The Camp Bucca has held as many as 14,000 detainees over the years, including the most dangerous in U.S. custody.

In total, around 100,000 people have been detained by U.S. forces since the 2003 U.S. invasion.

Following Bucca's closure, around 8,300 detainees remain in U.S. custody in Iraq, either in Camp Cropper near Baghdad airport or at Camp Taji north of the Iraqi capital.

Iraqi officials say some who have been freed have returned to violence.

They've been vetted as some of the most dangerous threats not only to Iraq but internationally, said Lt. Col. Kenneth King, the commander of the Bucca detention facility.

U.S. commanders say they expect to transfer all detainees to Iraqi authorities in January.